This invention relates to a method of and apparatus for cutting tubing, and, more particularly, to a system employing a chemical cutting fluid of highly reactive incendiary character such that when brought into contact with tubing suspended in a wellbore, it will quickly burn therethrough. The term "cutting" is used herein as a generic term to include cutting, severing, perforating or slotting of tubing and other objects, as well as their complete disintegration. The objects referred to may be metal pipe or wellbore lining, including the earth formation surrounding or forming the wall of the wellbore or extraneous foreign objects such as lost drilling tools suspended therein. Prior apparatus for cutting tubing are commonly referred to simply as "tubing cutters". Such cutters are used, for example, in drilling operations where it is desired to detach a bullplug or other obstruction from the lower end of a tubing in a wellbore by severing the tubing above the bullplug. Tubing cutters are also used to salvage the tubing from an abandoned well or the part of the tubing above a stuck point.
There are various types of prior art tubing cutters, for example, shaped charged cutters and chemical cutters. There are problems associated with shaped charge cutters, however, because a tubing string into which the cutter can be lowered into the wellbore, often contains a pump seating nipple or landing nipple which forms a partial constriction in the tubing by reducing the size of the bore. This landing nipple may be located at a point intermediate the surface and the part of the tubing to be cut. In tubing cutters which are constructed to be partially recovered, the force generated by an explosion of a shaped charge in the tubing cutter causes parts of the cutter which are not disintegrated to become expanded or the end of the tubing to become flared. This expansion often times prevents the retrieval of the expanded part.
Chemical cutters have been shown to be very reliable while overcoming many of the aforesaid problems. They have been referred to as the simplest and most efficient tool for tubing cutting because the cut is made without flare, debris or damage to the adjacent string of tubing. One such prior art construction is shown and described in U.S. Pat. Nos. 2,918,125, issued Dec. 22, 1959 and 3,076,507, issued Feb. 5, 1963, both to William G. Sweetman. As set forth in the Sweetman Patents, the cutting fluids employed in accordance with that invention are fluids which are extremely active chemically and which when brought into contact with most oxidizable substances, react violently therewith generating extremely high temperatures sufficient to melt, cut or burn the adjacent tubing. Fluids such as halogenflourides, including cholorine tri-flouride as well as bromine tri-flouride, have been proven to be effective in tubing cutting.
There are several considerations which remain of major import in the utilization of such chemical cutters as set forth above. First, the secure anchoring, or downhole positioning of the cutter within the wellbore is a major consideration. The cutter must be fixedly positioned within the wellbore so as to make a uniform cut and to prevent the flaring or creation of debris within the wellbore to frustrate the attempt of an effective cutting operation. The rigidity of the anchoring and centralized positioning is thus of critical import. It is also necessary that the overall unit be functionally reliable, including sure ignition, positive fluid seals and environmental durability. For example, the insertion of the tool into the wellbore is coincident with immersion in those fluids normally present therein such as mud and sand. It is thus important that such substances do not adversely affect the operation of the subject tooling.
It would be an advantage therefore to provide a chemical tubing cutter which would overcome many of the problems of the prior art wherein the tubing cutter could be reliably and securely positioned within the wellbore for the cutting operation. It would be a further advantage to provide a single tubing cutter securable within known conduits of a different size while effecting the equivalent reliability. The method and apparatus of the present invention includes such a chemical tubing cutter wherein a multi-angulated slip assembly is utilized for circumferential engagement within tubing of different sizes. The tubing cutter of the present invention also provides a means for the reliable sealing, ignition and actuation of the incendiary elements therein for the proper maintenance and control thereof.